Substitute Marouane Fellaini scored a last-minute winner - his first goal in a United shirt - to get Van Gaal off to a winning start at Old Trafford.

Despite taking a sixth win from as many matches in charge of the club, Van Gaal feels there is still plenty of room for improvement with United set to kick off their Premier League campaign at home to Swansea on Saturday.

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"We have won in spite of that we have played our worst game," Van Gaal told MUTV. "We made a lot of wrong choices, wrong passes and we didn't press too much."

Van Gaal was happier with the spirit United showed. While Wayne Rooney hit the post with a penalty, Darren Fletcher put them ahead and, after Rodrigo Moreno equalised for Valencia, Fellaini came off the bench to score in his first appearance under Van Gaal.

"We have dominated the game, only in 15 minutes in the second half we are not dominating, I think, and at the end we are the winner because we are fighting until the last minute and that is also a good signal, I believe," Van Gaal continued.

Ahead of Saturday's game, Van Gaal is concerned he will be without central defender Jonny Evans and wing-backs Luke Shaw and Antonio Valencia, who all missed the Tuesday's win.

"We have a lot of injuries now," he added. "Evans was injured, Shaw was injured, Valencia was injured and they are three defenders so we have to wait and see if they can play on Saturday."

Fellaini and Adnan Januzaj, both part of the Belgium squad that reached the last eight of the World Cup, came on for the final quarter of an hour, but Van Gaal said the duo are not yet fit enough to play any longer.

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"It is difficult to imagine Manchester United having another season like last time out under Louis van Gaal. It's easy to laud a manager through past stories but contemporary examples back up the point that Van Gaal could be United's biggest strength."

"Fellaini and Januzaj have only [had] one week, one-and-a half weeks to train with us so they cannot make more minutes," he said.

Van Gaal thanked the United supporters after being given a rapturous reception when he emerged before kick-off, but again warned he cannot provide a quick-fix for all of the club's problems.

"Fantastic," he added of the experience. "I come on the pitch and they are shouting and screaming your name. It is also a big pressure because they expect a lot and you cannot change everything in three or four weeks and the players cannot change either so we have to wait and see and develop and we need time but we have won every game until now and that is fantastic."

United defender Phil Jones was appreciative of the test that Valencia offered at Old Trafford, particularly given that the squad are still adapting to Van Gaal's new 3-5-2 system.

"It was a great game for us to test ourselves against a top Valencia team," Jones said. "At times they caused us a few problems but we worked well and battled well. We could've played better in stages but we deserved to win.

"It's been a great pre-season for us. We're learning a new system, we're still learning but we're enjoying it. We've capped off a good pre-season and we're looking forward to the start of the season now.

"It's the boss' first game here and the first game for a few of the young lads too. I thought they were terrific tonight and that only stands us in good stead for the season."

After enduring a tricky start to his Manchester United career, perhaps it is fair that Marcos Rojo celebrated so boisterously as he watched his first professional club Estudiantes beat fierce rivals Gimnasia